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This is an old E6420, so no longer under warranty.

I did some searching on YouTube and it looks like I would have to take the laptop completely apart to find the fault.   No thanks! So far the screen problem has not recurred. If it does and gets worse, I'll get a new used laptop and move my stuff to it.

laptop screen stems from the backlighting of the display turning on and off. This can result from multiple factors, but the most common cause is the wearing out of the backlight inverter.

Thanks for the help. So far the problem has not recurred in 5 days, so I'm keeping my fingers crossed and taking extra precautions not to drop the laptop.

After about 2 months the problem recurred. I didn't drop it or anything. It was just sitting there, running with the screen open, and when I went to use it the screen was dark, then as I moved the mouse or the lid, it would switch around and would change back and forth between the dark screen and yellow background, but hardly long enough to get anything done. Now, after about an hour it seems to be almost back to normal, fingers crossed. Screen occasionally flashing to yellow background, but not as bad as before. I did notice it also felt very warm where the fan exhausts. Now it's pretty much normal, core temperature 50°C +/- 5°, which is about normal in my experience. Could the inexplicable temperature rise affect the screen?

Well, it ended up going back to normal, but last week the laptop slid off my tray, took a bump and the screen was bad again. I struggled with an external monitor, but decided I needed an upgrade because this old beast was still running Windows 7.  I now have a new (to me) Dell Latitude 5520 with all my stuff transferred to a 2.0TB SSD running Windows 10.